1. Field of the Invention
The present invention relates to a method for temperature compensation of a voltage-controlled crystal oscillator in a phase locked loop circuit, in which the digital output signals of the crystal oscillator are compared to digital reference clock signals with respect to phase and are supplied to a processor which is equipped with a memory.
2. Description of the Related Art
The individual system components in digital communications networks, such as switching and transmission equipment, are equipped with clock generators. These clock generators provide time control of the internal operations or procedures of the system components. In order to achieve frequency synchronism between the individual system components, clock information generated by a central, high-precision clock device is communicated to the system components via the existing transmission links and transmission equipment and is used for synchronization of the implemented clock generators. Such frequency synchronization devices are known as phase frequency control circuits, referenced below as phase locked loop (PLL) devices.
A PLL device essentially contains a voltage-controlled oscillator and a phase comparator with a following filter. The comparator compares the output signals of the oscillator to the clock information in terms of their phases and controls the oscillator depending on the result of the comparison. The oscillator is predominantly in the form of a crystal oscillator, whereby the properties of the crystal that critically influence the precision of an oscillator are taken into consideration. In greater detail, these properties are its temperature dependency and the modification of the crystal properties that are produced by aging. The temperature dependency of the crystal results in temperature fluctuations of the crystal caused by changes in the ambient temperature producing deviations in the frequency of the oscillator. The frequency deviations, which are dependent on crystal temperature, follow temperature response curves that are permanently assigned to each crystal. The temperature dependency of the crystal is essentially dependent on the type of crystal section, i.e. the angle relative to a crystal axis at which the crystal is cut from a quartz crystal. Since the frequency range of the PLL devices in digital telecommunications network components usually lies above 1 MHz, crystals having what are referred to as "AT sections" are used.
Methods are known with which the frequency deviations that are produced by temperature fluctuations can be compensated. Such a method is described in the ZVEI publication, Schwingquarze, ein unverzichtbares Bauelement in der Elektronik, Conference Documentation of the Quartz Symposium, 1985. The crystal is thereby thermically coupled to a temperature sensor of a temperature measuring device. The measured values which are present in analog form are digitized in an analog to digital (A/D) converter and are supplied to a memory in which the voltage values required for compensation are stored in digital form. Each digitized measured value has a digital compensation value assigned thereto. After a digital to analog (D/A) conversion, the compensation value is fed to the voltage control input of the oscillator as an analog compensation voltage and effects a correction of the temperature-conditioned frequency deviations. The frequency deviations of the oscillator produced due to temperature fluctuations can therefore be compensated with this digital compensation method.
An oscillator constructed in this manner can also be utilized in a PLL device. When so used, it is thereby to be considered particularly advantageous to replace the memory with a processor which is equipped with a memory, since, in addition to the temperature compensation of the crystal, the processor can perform correction of the oscillator, such correction being required due to the frequency deviations or, respectively, phase deviations of the two clock signals.
It is also known not to assign a compensation value in the memory to each temperature step defined by the A/D converter. The appropriate compensation values are stored for a selected plurality of crystal temperatures within the selected range of operating temperature, for example -20.degree. C. through +70.degree. C. The plurality of crystal temperatures for which compensation values are stored is based, for example, on the curvature of the temperature response curve of the crystal, since the compensation values for the crystal temperatures currently measured that lie between the selected crystal temperatures are calculated by linear interpolation. This basis on the curvature is required due to the pronounced curvatures of the temperature response curve of more selected crystal temperatures in order to keep the interpolation errors low. The respective intermediate value is thereby to be calculated for each voltage step defined by the A/D converter.